Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

Tangential probes should therefore only be used when flaws in one direction
only are sought. One of their common applications is for the detection of
fatigue cracks in the bead seat radius of aircraft wheels, which always occur
in a circumferential direction. Tangential probes allow the whole of the bead
seat area to be tested in one scan, unlike standard surface probes, which
require a number of scans because a small diameter coil needs to be used to
reliably detect short cracks. When a tangential probe is used, a short crack in
the bead seat radius will always distort the eddy currents, although since the
distortion occurs only at one location in the eddy current path, the relative
effect on the eddy currents is less than for a standard surface probe, where, if
the coil is immediately above the crack, most of the eddy current field is
disrupted. This means that the crack signal will not be as strong as that for a
small pencil probe, however, the technique shows adequate sensitivity, and is
used by a number of major airlines with a consequent saving in either
operator costs, or the cost of an automated wheel scan system.
Charlie Chong/ Fion Zhang